Digital pulse width modulator for use in electrostatic printing mechanisms

Abstract

A fully digital pulse width modulator substantially doubles resolution in a laser printer by outputting data to the laser on both the rising and falling edges of the clock cycle. A counter and the clock itself are used to select input to a multiplexer, and consequently, the data output to the laser from the multiplexer. A data selector code, generated by concatenating the binary value of the counter and the inverted clock bitwise, selects which of the 16 bits representing a pixel to place onto the data line, so that all 16 bits are output to the laser serially and sequentially in eight clock cycles. By using both the rising and falling edges of a clock cycle, the clock speed of the device is effectively doubled, without increasing actual clock speed. Device resolution is improved simply and inexpensively without major modification of printed circuit boards.

Description

BACKGROUND OF THE INVENTION[0001]1. Technical Field[0002]The invention generally relates to image forming apparatuses such as laser printers. More particularly, the invention relates to a system and method for increasing resolution of a laser printer through the use of a digital pulse width modulator that clocks digital data specifying grayscale values of pixels to be printed to the laser on both the ascending and descending edges of the clock, effectively doubling the clock rate and thereby increasing the resolution of the printer.[0003]2. Technical Background[0004]A typical laser printer usually includes an electrostatic printing mechanism composed of a cylindrical drum having an electrically charged surface. Toner particles of opposite charge adhere to the drum. The image to be printed is formed on the drum by means of a laser beam directed toward the drum. Wherever the laser impinges on the drum, the drum surface is discharged, creating an area in which the charged toner particl...

AI Technical Summary

Benefits of technology

[0008]The invention provides a fully digital pulse width modulator in an electrostatic printing mechanism of a laser printer that outputs data to the laser on both the rising and falling edges of the clock cycle. Thus, the clock rate is effectively doubled, consequently doubling resolution of the laser printer. The digital pulse width modulator of the current invention includes a multiplexer and a counter in combination with the clock itself to select input to the multiplexer and, consequently, the data output to the laser from the multiplexer. In a preferred embodiment of the invention, each pixel is specified by a 16-bit value. The 16 bits are applied to the data inputs of a 16:1 multiplexer. The counter increments one for each clock cycle, up to eight clock cycles. The binary value of the counter is concatenated bitwise with the binary value of the inverted clock to generate a 4-bit data selector code that is input to the multiplexer. The data from the data input corresponding to the data selector code is input to the multiplexer and subsequently output to the laser. In this way, the 16 bits representing each pixel of the image are output serially and sequentially to the laser, in only eight clock cycles. Because the invention makes use of both the rising and falling edges of a clock cycle, the clock speed of the device is effectively doubled, without increasing the actual clock speed. By using 16 bits to represent each pixel, the resolution of the device is also effectively doubled. The invention provides a simple, inexpensive way to improve the resolution of a laser printer, without resort to major modification of printed circuit boards.

Problems solved by technology

However, images, such as photographs, have fuzzy edges and gradations in tone.

Such analog pulse width modulators, however, suffer several disadvantages.

Due to their analog nature, they are inherently sensitive to noise and they are vulnerable to voltage drifts and temperature drifts, requiring frequent recalibration.

Furthermore, they are implemented using discrete components, rendering them complicated and expensive.

A deficiency of this type of arrangement is that the clock speed imposes an upper limit on the granularity, or resolution that can be achieved, thus limiting the image quality. J. Hewes, Method of increasing the grayscale resolution of a non-impact LED page printer, U.S. Pat. No. 5,105,202 (Apr. 14, 1992) describes such a system and suggests that resolution can be improved by increasing the data output of the shift register.

However, no means for increasing the shift register's output is suggested.

Increasing the clock speed to achieve a greater output is not a practical or feasible solution.

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